1. Field of the Invention
The present invention relates generally to a video-phone system and more particularly to a video-phone or television telephone system that can transmit and receive still pictures over existing telephone lines or special telephone lines.
2. Description of the Prior Art
Video-phone or television telephone systems using telephone lines have been previously proposed. According to the conventional television telephone system, a video signal, for example, a picked-up image signal is processed so that it can be transmitted over standard telephone lines and the thus processed video signal is transmitted over the telephone lines in place of telecalls. In this case, during transmission of one of the above-mentioned video signal or the audio signal from the telephone set a change-over can occur by driving a switching relay of the telephone lines in association with predetermined transmitting operations. During reception, the video signal and the audio telephone signal must be automatically changed-over by using a key signal which is detected from the transmitted signal.
In the above-mentioned conventional television telephone system, however, a detecting circuit for detecting the key signal must be connected to the telephone set and to the telephone lines used in the communication system. There is a risk that a part of the audio signal used for communication will be mis-detected as the key signal. Should mis-detection occur in the above-mentioned television telephone system, the television-telephone system will be switched to the receiving mode, and the telephone communication will be interrupted. This is very undesirable. Therefore, detection of the key signal must be accomplished with great care.
Telephones have been used with automatic answering functions. This is called an automatic answering telephone. In an automatic answering telephone during use, a user can remotely control the automatic answering telephone which is installed in the user's home by continuously transmitting to it from a remote location a signal which has a predetermined frequency for a predetermined period of time. In this case, however, in order to avoid mis-detection, the selected predetermined duration must be more than several seconds. Also, in the conventional television telephone system, the transmission of the video signal causes telephone communication to be interrupted for a considerable period. It can be realized from the foregoing description that it is undesirable to interrupt telephone communication for long periods.
As described above, according to the conventional television telephone system, if the key signal is mis-detected, telephone communication is interrupted which is undesirable. Further, to prevent mis-detection of the key signal, the time during which telephone communication is interrupted is increased.